This invention relates to a process for the production of esters by the direct addition of a saturated lower carboxylic acid to an olefin in the presence of a catalyst and to a method of purifying the recycle streams to the process to prolong catalyst activity and catalyst life.
It is well known to produce esters such as ethyl acetate or n-butyl acetate by an esterification reaction of ethanol or n-butanol respectively with acetic acid in the presence of an acidic catalyst. One such method is described in GB-A-1438410. It is also known to produce esters such as these by the addition of an acid to an olefin. The process for making ethyl acetate by the direct addition of acetic acid to ethylene produces a range of by-products and impurities. While some of the main by-products are similar to those removed from ethyl acetate made by esterification of acetic acid (eg ethanol and diethyl ether), the process is intrinsically different and therefore produces a range of quite different by-products as impurities. More specifically, because ethylene is one of the reactants there is scope for oligomerisation thereof resulting in a range of by-products including inter alia hydrocarbons ranging from ethane and butane through to longer chain hydrocarbons of 10-12 or more carbon atoms. These by-products can include both saturated and unsaturated hydrocarbons and may also contain oxygenated compounds. It is known that slowly with time during this direct addition reaction, some deactivation of catalyst is observed and one of the reasons for this was believed to be the formation of coke derived from aromatic and olefinic materials. However, to date no aromatics have so far been detected in the product streams of the direct addition reaction.
In order to minimise this catalyst deactivation, it was decided to use a product purification process involving steps of distillation and recycle during the direct addition process. The distillation scheme primarily comprises an initial column to which the liquid addition reaction products are fed (via flash separators) and from which excess/unreacted acetic acid and heavy ends are removed, a second column from which the light ends, ethanol and water are removed to generate a crude ethyl acetate product, a third column in which the alcohol (and any ethyl acetate) are separated from water in order to enable recovery of the alcohol for recycling and a fourth column in which the crude ethyl acetate from the second column is refined and purified by removal of any residual by-products passed overhead of the second column as azeotropes with eg water and the medium ends which are purged from the base of this column. However, the designs used hitherto had no provision for the removal of aldehyde by-products (which are known to cause catalyst deactivation) formed during the reaction. The reason for this is that it had always been believed that the deactivation of the catalyst was due to coking of aromatics or olefins as stated above.
It has now been found that for ethyl acetate produced in a direct addition reaction, the distillation scheme can be tailored not only to remove a new range of impurities characteristic to the direct addition process but also to ensure that the recycle streams are of a sufficient quality thereby avoiding the impairment of catalyst activity or lifetime.
Accordingly, the present invention is a process for the purification of the reaction products of a direct addition reaction comprising reacting ethylene with acetic acid in the presence of a catalyst to form ethyl acetate and purifying the recycle streams, the purification process comprising the steps of:
(i) feeding the reaction products to an acid removal column (A) where acetic acid is removed from the base of the column, at least a fraction comprising light ends comprising inter alia hydrocarbons, ethyl acetate, ethanol, diethyl ether and water are withdrawn overhead and fed into a decanter (A1) for separation of said overheads into an ethyl acetate-rich phase and an aqueous (water-rich) phase, PA0 (ii) recycling at least a portion of the ethyl acetate-rich phase and substantially all of the aqueous phase from decanter (A1) as reflux separately back to column (A) at or near the top thereof, PA0 (iii) feeding the remainder of the ethyl acetate-rich phase from decanter (A1) to a refining column (C) at or near the top thereof, PA0 (iv) removing from column (C): PA0 (v) removing a purge comprising acetaldehyde at or near the top of the aldehyde removal column and recycling the diethyl ether recovered from the base of the aldehyde removal column to the esterification reactor, and PA0 (vi) purifying the refined ethyl acetate in column (E).
(a) a base product comprising substantially refined ethyl acetate which is fed to a purification column (E); PA1 (b) an overhead product comprising light ends including inter alia acetaldehyde and diethyl ether which is fed to an aldehyde removal column; and PA1 (c) a side-draw comprising primarily ethyl acetate, ethanol and some water which is removed at a point below the feed point of the ethyl acetate-rich phase from column (A),
The catalyst used for this addition reaction is suitably an acid catalyst and may be selected from phosphoric acid, phosphonic acid and a heteropolyacid catalyst. It is preferably a heteropolyacid catalyst which may be supported on a carrier. The carrier, where used is suitably silica which may be in any shape or form selected from beads, agglomerates, globules, pellets, extrudates and granules.
Specific examples of such silicas include, but are not limited to the Degussa 350 silica (ex Degussa) and Grace 57 grade silicas (ex W R Grace). The reaction may be carried out using commercial acetic acids and may include an amount of diethyl ether which is recovered from the reaction products and is recycled to this step.
The Table below shows a typical composition of the reaction products of a direct addition reaction which is used as feed to the acid removal column (A).
Depending on the age of the catalyst and the way the process is operated the relative concentration of the components changes Table I below gives a view that is currently observed.
TABLE 1 Component Concentration by weight Ethyl acetate 46.95 Acetic acid 38.35 Water 11.41 Ethanol 1.83 Diethyl Ether (DEE) 0.59 Acetaldehyde 0.03 Light Ends 0.91 Medium Ends 0.06 Heavy Ends 0.01